FIELD OF THE INVENTION
The invention relates to a waste-heat steam generator, in particular for a combined-cycle power plant, with a number of heating surfaces heatable by a flue gas and carrying a medium to be heated. More specifically, the generator contains a preheater following an evaporator on the flue gas side and a superheater precedes the evaporator on the flue gas side.
Such a waste-heat steam generator or waste-heat boiler is usually part of a combined-cycle power plant, in which the heat contained in the expanded working medium from the gas turbine is used to generate steam for the steam turbine. The heat is transmitted by a number of heating surfaces which are disposed in the form of tubes or tube bundles in the waste-heat steam generator. These, in turn, are connected into the water/steam circuit of the steam turbine, the circuit containing at least one pressure stage. In this case, each pressure stage usually has, as heating surfaces, a preheater or economizer and a evaporator as well as a superheater. A connection having a first or high-pressure stage and a second or low-pressure stage--a so-called two-pressure process--is known from European Patent EP 0 410 111 B1.
Due to the further development of the stationary gas turbine towards higher unit output and higher efficiency, in conjunction with an increased exhaust-gas temperature at the gas turbine outlet, it can be seen that it would also become necessary to adapt the fresh-steam parameters, namely the fresh-steam temperature and fresh-steam pressure, in order to achieve a further increase in plant efficiency. An increased exhaust-gas temperature of the gas turbine consequently results in an enhancement of the fresh-steam parameters, in order to achieve correspondingly high efficiency of the overall combined-cycle process.
In a waste-heat steam generator of this type, configured as a drum boiler and working on the circulation principle, evaporation is ended by separating the steam from the not yet evaporated water at deflecting plates in the drum. This separation requires circulation. The circulation and the steam separation make it necessary to have, between the water and steam, an appreciable density difference which, in turn, presupposes an appreciable pressure deviation (&gt;50 to 60 bar) from the critical pressure (221 bar). Steam separation thus places an upper limit on the pressure, so that a drum boiler can work only below an operating pressure of 160 to 170 bar. Moreover, high steam pressures necessitate large wall thicknesses of the water/steam separating drum, thus sharply restricting the permissible rate of temperature change in the start-up and load-alternation modes undesirably.
In the forced once-through principle, such as a fossil-fuelled once-through steam generator, as an alternative to the circulation or natural circulation principle, the heating of the evaporator tubes forming a combustion-chamber wall leads to complete evaporation of the flow medium in the evaporator tubes in a single pass. In this forced once-through principle, the end of evaporation and consequently, at the same time, the commencement of steam superheating occur as a function of the load and are not locally fixed. In this case, with a once-through steam generator of this type, a freshsteam pressure above the critical water pressure may be implemented due to the absence of steam separation or water precipitation. A once-through steam generator of this type is known, for example, from European Patent EP 0 595 009 B1.
However, in conjunction with a gas turbine in a so-called combined-cycle power plant, a once-through steam generator of this type is usually not employed as a pure waste-heat steam generator. Instead, the oxygenous exhaust gas from the gas turbine serves merely as combustion air for the firing system of this fossil-fuelled steam generator. Disadvantages of a once-through steam generator of this kind are its high outlay in terms of connection and regulation and its high start-up losses due to so-called water ejection. This occurs when evaporation in the evaporator commences and the steam pushes out the downstream water quantity (water drops). Additional separating and monitoring components, provided in order to overcome the start-up losses caused thereby, additionally increase the technical outlay and consequently the investment costs which increase sharply with a desired implementation of high and very high steam pressures. Another essential disadvantage of the fired once-through steam generator, as compared with the pure waste-heat steam generator, is the comparatively uneven adaptation of the temperature profile of the heated medium (water/water-steam line) to the temperature profile of the heating medium (flue-gas line).